The increased demands placed on an aircraft flight crew as a result of more complex technology, ever increasing aircraft traffic, and increased demands for safety has brought about a requirement for monitoring aircraft traffic in the vicinity of an aircraft. Such monitoring includes the automatic identification of potential threats to a surveillance aircraft monitoring target aircraft in such vicinity. As a result, aircraft have transponders which in response to appropriate interrogation signals generate reply signals that may provide information with respect to the range, altitude and bearing of the target aircraft. Certain traffic control system transponders, such as the Mode S system include unique aircraft identifiers so that each aircraft is interrogated separately and each reply is stamped with the identity of the target aircraft. This significantly simplifies surveillance processing by the surveillance aircraft.
In systems such as the Air Traffic Control Radar Beacon System, (ATCRBS), which do not include unique aircraft identifiers in reply to interrogation signals, the determination of tracks representative of target aircraft from the processing of such replies is more difficult. The information obtained from the replies provided by the target aircraft in response to periodic interrogation of target aircraft by the surveillance aircraft during surveillance periods may contain range, altitude and bearing information. Such information is subjected to algorithms in a TCAS to provide a target aircraft track. Once a track is identified and initialized, then the track can be updated and stabilized while continually monitoring the track to determine if the target aircraft represented by the track is a threat to the surveillance aircraft.
Track determination is complicated for several reasons generally involving multipath and spurious target aircraft replies. A surveillance aircraft transmits an interrogation signal to target aircraft whereupon a transponder in the target aircraft provides a reply signal containing encoded data. Numerous replies from a single target aircraft can be received by the surveillance aircraft due to ground reflection and other replies may be false due to electromagnetic interference or other effects. Three replies in consecutive surveillance periods meeting various requirements, such as range requirements and altitude requirements, are utilized to initiate a track. In the case of Mode S transponder equipped target aircraft, such Mode S aircraft are identified by reception of squitters therefrom. If the surveillance aircraft received squitters indicating that the new target aircraft should be tracked, then interrogations are sent by the surveillance aircraft and replies are provided by the target aircraft with identifiers to provide for tracking. Therefore, target aircraft track initialization processes utilizing reply information as opposed to Mode S identifiers relate only to the initialization of tracks for target aircraft utilizing ATCRBS transponders.
One track initialization method which relates only to the altitude requirements for initializing an altitude track for target aircraft is suggested in Minimum Operational Performance Standards (MOPS) for Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment manual document No. DO-185 by the Radio Technical Commission for Aeronautics (RTCA) which governs the operation of aircraft collision avoidance apparatuses. The MOPS method includes use of an altitude correlation table to determine whether three replies from three consecutive surveillance periods correlate such that the altitude requirements for initializing a track are satisfied and for determining the initial altitude of such a track if initialized. The replies contain eleven bits of Gilham altitude data. The three altitude replies are said to correlate if they meet certain requirements implying that they are likely to be from the same intruder. The eleven bits of Gilham altitude data are used to determine the altitude from each reply of a target aircraft. Each altitude bit from each reply also has an associated confidence level, either high or low.
The MOPS initialization table groups corresponding altitude bits from each of the three replies, one from each surveillance period, along with their confidence levels to produce a table of 64 possible output combinations of altitude bits and bit agreement. The 11 bits are D2, D4, A1, A2, A4, B1, B2, B4, C1, C2, and C4. If the table produces bit agreement on all eight D, A and B bits or if it produces bit agreement on seven of the D, A and B bits and on at least one of the C bits, then the three replies correlate and according to the altitude requirements, a track may be initialized if other requirements such as range requirements are satisfied. Since each group of three bits, one from each reply, are handled independently of the other ten bits in the Gilham altitude code, strange altitude initializations occur. In addition, strange initial altitudes for such tracks are created from the replies which are said to correlate. For example, in some cases the initial altitude of the track is set at or near the altitude of one of the replies which is significantly different than the other two replies which have matching altitudes.
These false altitude initializations may lead to false tracks being created representative of target aircrafts. Such false tracks may lead to false traffic advisories or resolution advisories. When such traffic advisories or resolution advisories are displayed to the flight crew on a traffic advisory or resolution advisory display, these false tracks become a distraction to the flight crew and can cause an unsafe maneuver comand. Therefore, there is a need for a improvement in altitude track initialization methods to reduce the number of false altitude track initializations.